This section describes builtin commands which are unique to or have been extended in Bash. Some of these commands are specified in the posix standard.
alias alias [-p] [name[=value] ...]
Without arguments or with the -p option, alias prints
the list of aliases on the standard output in a form that allows
them to be reused as input.
If arguments are supplied, an alias is defined for each name
whose value is given. If no value is given, the name
and value of the alias is printed.
Aliases are described in Aliases.
bind bind [-m keymap] [-lpsvPSV]
bind [-m keymap] [-q function] [-u function] [-r keyseq]
bind [-m keymap] -f filename
bind [-m keymap] -x keyseq:shell-command
bind [-m keymap] keyseq:function-name
bind readline-command
Display current Readline (see Command Line Editing) key and function bindings, bind a key sequence to a Readline function or macro, or set a Readline variable. Each non-option argument is a command as it would appear in a Readline initialization file (see Readline Init File), but each binding or command must be passed as a separate argument; e.g., ‘"\C-x\C-r":re-read-init-file’.
Options, if supplied, have the following meanings:
-m keymapemacs,
emacs-standard,
emacs-meta,
emacs-ctlx,
vi,
vi-move,
vi-command, and
vi-insert.
vi is equivalent to vi-command;
emacs is equivalent to emacs-standard.
-l-p-P-v-V-s-S-f filename-q function-u function-r keyseq-x keyseq:shell-commandREADLINE_LINE variable to the contents of the Readline line
buffer and the READLINE_POINT variable to the current location
of the insertion point.
If the executed command changes the value of READLINE_LINE or
READLINE_POINT, those new values will be reflected in the
editing state.
The return status is zero unless an invalid option is supplied or an
error occurs.
builtinbuiltin [shell-builtin [args]]
Run a shell builtin, passing it args, and return its exit status.
This is useful when defining a shell function with the same
name as a shell builtin, retaining the functionality of the builtin within
the function.
The return status is non-zero if shell-builtin is not a shell
builtin command.
callercaller [expr]
Returns the context of any active subroutine call (a shell function or
a script executed with the . or source builtins).
Without expr, caller displays the line number and source
filename of the current subroutine call.
If a non-negative integer is supplied as expr, caller
displays the line number, subroutine name, and source file corresponding
to that position in the current execution call stack. This extra
information may be used, for example, to print a stack trace. The
current frame is frame 0.
The return value is 0 unless the shell is not executing a subroutine
call or expr does not correspond to a valid position in the
call stack.
commandcommand [-pVv] command [arguments ...]
Runs command with arguments ignoring any shell function
named command.
Only shell builtin commands or commands found by searching the
PATH are executed.
If there is a shell function named ls, running ‘command ls’
within the function will execute the external command ls
instead of calling the function recursively.
The -p option means to use a default value for PATH
that is guaranteed to find all of the standard utilities.
The return status in this case is 127 if command cannot be
found or an error occurred, and the exit status of command
otherwise.
If either the -V or -v option is supplied, a
description of command is printed. The -v option
causes a single word indicating the command or file name used to
invoke command to be displayed; the -V option produces
a more verbose description. In this case, the return status is
zero if command is found, and non-zero if not.
declaredeclare [-aAfFilrtux] [-p] [name[=value] ...]
Declare variables and give them attributes. If no names are given, then display the values of variables instead.
The -p option will display the attributes and values of each name. When -p is used with name arguments, additional options are ignored.
When -p is supplied without name arguments, declare
will display the attributes and values of all variables having the
attributes specified by the additional options.
If no other options are supplied with -p, declare will
display the attributes and values of all shell variables. The -f
option will restrict the display to shell functions.
The -F option inhibits the display of function definitions;
only the function name and attributes are printed.
If the extdebug shell option is enabled using shopt
(see The Shopt Builtin), the source file name and line number where
the function is defined are displayed as well.
-F implies -f.
The following options can be used to restrict output to variables with
the specified attributes or to give variables attributes:
-a-A-f-i-l-r-ttrace attribute.
Traced functions inherit the DEBUG and RETURN traps from
the calling shell.
The trace attribute has no special meaning for variables.
-u-xUsing ‘+’ instead of ‘-’ turns off the attribute instead,
with the exceptions that ‘+a’
may not be used to destroy an array variable and ‘+r’ will not
remove the readonly attribute.
When used in a function, declare makes each name local,
as with the local command. If a variable name is followed by
=value, the value of the variable is set to value.
The return status is zero unless an invalid option is encountered,
an attempt is made to define a function using ‘-f foo=bar’,
an attempt is made to assign a value to a readonly variable,
an attempt is made to assign a value to an array variable without
using the compound assignment syntax (see Arrays),
one of the names is not a valid shell variable name,
an attempt is made to turn off readonly status for a readonly variable,
an attempt is made to turn off array status for an array variable,
or an attempt is made to display a non-existent function with -f.
echoecho [-neE] [arg ...]
Output the args, separated by spaces, terminated with a
newline.
The return status is always 0.
If -n is specified, the trailing newline is suppressed.
If the -e option is given, interpretation of the following
backslash-escaped characters is enabled.
The -E option disables the interpretation of these escape characters,
even on systems where they are interpreted by default.
The xpg_echo shell option may be used to
dynamically determine whether or not echo expands these
escape characters by default.
echo does not interpret -- to mean the end of options.
echo interprets the following escape sequences:
\a\b\c\e\f\n\r\t\v\\\0nnn\xHHenableenable [-a] [-dnps] [-f filename] [name ...]
Enable and disable builtin shell commands.
Disabling a builtin allows a disk command which has the same name
as a shell builtin to be executed without specifying a full pathname,
even though the shell normally searches for builtins before disk commands.
If -n is used, the names become disabled. Otherwise
names are enabled. For example, to use the test binary
found via $PATH instead of the shell builtin version, type
‘enable -n test’.
If the -p option is supplied, or no name arguments appear, a list of shell builtins is printed. With no other arguments, the list consists of all enabled shell builtins. The -a option means to list each builtin with an indication of whether or not it is enabled.
The -f option means to load the new builtin command name from shared object filename, on systems that support dynamic loading. The -d option will delete a builtin loaded with -f.
If there are no options, a list of the shell builtins is displayed.
The -s option restricts enable to the posix special
builtins. If -s is used with -f, the new builtin becomes
a special builtin (see Special Builtins).
The return status is zero unless a name is not a shell builtin
or there is an error loading a new builtin from a shared object.
helphelp [-dms] [pattern]
Display helpful information about builtin commands.
If pattern is specified, help gives detailed help
on all commands matching pattern, otherwise a list of
the builtins is printed.
Options, if supplied, have the following meanings:
-d-m-sThe return status is zero unless no command matches pattern.
letlet expression [expression]
The let builtin allows arithmetic to be performed on shell
variables. Each expression is evaluated according to the
rules given below in Shell Arithmetic. If the
last expression evaluates to 0, let returns 1;
otherwise 0 is returned.
locallocal [option] name[=value] ...
For each argument, a local variable named name is created,
and assigned value.
The option can be any of the options accepted by declare.
local can only be used within a function; it makes the variable
name have a visible scope restricted to that function and its
children. The return status is zero unless local is used outside
a function, an invalid name is supplied, or name is a
readonly variable.
logoutlogout [n]
Exit a login shell, returning a status of n to the shell's
parent.
mapfile mapfile [-n count] [-O origin] [-s count] [-t] [-u fd] [
-C callback] [-c quantum] [array]
Read lines from the standard input into the indexed array variable array,
or from file descriptor fd
if the -u option is supplied.
The variable MAPFILE is the default array.
Options, if supplied, have the following meanings:
-n-O-s-t-u-C-cIf -C is specified without -c, the default quantum is 5000. When callback is evaluated, it is supplied the index of the next array element to be assigned as an additional argument. callback is evaluated after the line is read but before the array element is assigned.
If not supplied with an explicit origin, mapfile will clear array
before assigning to it.
mapfile returns successfully unless an invalid option or option
argument is supplied, array is invalid or unassignable, or array
is not an indexed array.
printfprintf [-v var] format [arguments]
Write the formatted arguments to the standard output under the
control of the format.
The format is a character string which contains three types of objects:
plain characters, which are simply copied to standard output, character
escape sequences, which are converted and copied to the standard output, and
format specifications, each of which causes printing of the next successive
argument.
In addition to the standard printf(1) formats, ‘%b’ causes
printf to expand backslash escape sequences in the corresponding
argument,
(except that ‘\c’ terminates output, backslashes in
‘\'’, ‘\"’, and ‘\?’ are not removed, and octal escapes
beginning with ‘\0’ may contain up to four digits),
and ‘%q’ causes printf to output the
corresponding argument in a format that can be reused as shell input.
The -v option causes the output to be assigned to the variable var rather than being printed to the standard output.
The format is reused as necessary to consume all of the arguments.
If the format requires more arguments than are supplied, the
extra format specifications behave as if a zero value or null string, as
appropriate, had been supplied. The return value is zero on success,
non-zero on failure.
readread [-ers] [-a aname] [-d delim] [-i text] [-n nchars] [-N nchars] [-p prompt] [-t timeout] [-u fd] [name ...]
One line is read from the standard input, or from the file descriptor
fd supplied as an argument to the -u option, and the first word
is assigned to the first name, the second word to the second name,
and so on, with leftover words and their intervening separators assigned
to the last name.
If there are fewer words read from the input stream than names,
the remaining names are assigned empty values.
The characters in the value of the IFS variable
are used to split the line into words.
The backslash character ‘\’ may be used to remove any special
meaning for the next character read and for line continuation.
If no names are supplied, the line read is assigned to the
variable REPLY.
The return code is zero, unless end-of-file is encountered, read
times out (in which case the return code is greater than 128), or an
invalid file descriptor is supplied as the argument to -u.
Options, if supplied, have the following meanings:
-a aname-d delim-e-i text-n ncharsread returns after reading nchars characters rather than
waiting for a complete line of input, but honor a delimiter if fewer
than nchars characters are read before the delimiter.
-N ncharsread returns after reading exactly nchars characters rather
than waiting for a complete line of input, unless EOF is encountered or
read times out.
Delimiter characters encountered in the input are
not treated specially and do not cause read to return until
nchars characters are read.
-p prompt-r-s-t timeoutread to time out and return failure if a complete line of
input is not read within timeout seconds.
timeout may be a decimal number with a fractional portion following
the decimal point.
This option is only effective if read is reading input from a
terminal, pipe, or other special file; it has no effect when reading
from regular files.
If timeout is 0, read returns success if input is available on
the specified file descriptor, failure otherwise.
The exit status is greater than 128 if the timeout is exceeded.
-u fdreadarray readarray [-n count] [-O origin] [-s count] [-t] [-u fd] [
-C callback] [-c quantum] [array]
Read lines from the standard input into the indexed array variable array, or from file descriptor fd if the -u option is supplied.
A synonym for mapfile.
sourcesource filename
A synonym for . (see Bourne Shell Builtins).
typetype [-afptP] [name ...]
For each name, indicate how it would be interpreted if used as a command name.
If the -t option is used, type prints a single word
which is one of ‘alias’, ‘function’, ‘builtin’,
‘file’ or ‘keyword’,
if name is an alias, shell function, shell builtin,
disk file, or shell reserved word, respectively.
If the name is not found, then nothing is printed, and
type returns a failure status.
If the -p option is used, type either returns the name
of the disk file that would be executed, or nothing if -t
would not return ‘file’.
The -P option forces a path search for each name, even if -t would not return ‘file’.
If a command is hashed, -p and -P print the hashed value,
not necessarily the file that appears first in $PATH.
If the -a option is used, type returns all of the places
that contain an executable named file.
This includes aliases and functions, if and only if the -p option
is not also used.
If the -f option is used, type does not attempt to find
shell functions, as with the command builtin.
The return status is zero if all of the names are found, non-zero
if any are not found.
typesettypeset [-afFrxi] [-p] [name[=value] ...]
The typeset command is supplied for compatibility with the Korn
shell; however, it has been deprecated in favor of the declare
builtin command.
ulimitulimit [-abcdefilmnpqrstuvxHST] [limit]
ulimit provides control over the resources available to processes
started by the shell, on systems that allow such control. If an
option is given, it is interpreted as follows:
-S-H-a-b-c-d-e-f-i-l-m-n-p-q-r-s-t-u-v-x-TIf limit is given, it is the new value of the specified resource;
the special limit values hard, soft, and
unlimited stand for the current hard limit, the current soft limit,
and no limit, respectively.
A hard limit cannot be increased by a non-root user once it is set;
a soft limit may be increased up to the value of the hard limit.
Otherwise, the current value of the soft limit for the specified resource
is printed, unless the -H option is supplied.
When setting new limits, if neither -H nor -S is supplied,
both the hard and soft limits are set.
If no option is given, then -f is assumed. Values are in 1024-byte
increments, except for -t, which is in seconds, -p,
which is in units of 512-byte blocks, and -n and -u, which
are unscaled values.
The return status is zero unless an invalid option or argument is supplied,
or an error occurs while setting a new limit.
unaliasunalias [-a] [name ... ]
Remove each name from the list of aliases. If -a is supplied, all aliases are removed. Aliases are described in Aliases.